Recent advances have enabled users of computer hardware to emulate large, permanently mounted, magnetic direct access storage device (DASD) volumes with a library of removable magneto-optical (MO) media. Such emulation offers the user substantial benefits, including lower cost per megabyte and reduced space requirements. For example, one permanently mounted volume of IBM Corporation's 3390 Model 2, a large DASD system employing ECKD (a trademark of the IBM Corporation) architecture, is formatted with 2,226 cylinders, each having 15 tracks at 56,664 bytes per track. During formatting, a home address (HA) and record 0 (R0) field is written to each track in the volume, a process which requires about four to five hours to complete (and includes a one-pass write procedure and no mount/demount delays).
Emulation of such a DASD can be achieved with a smaller, less expensive, automated data storage and retrieval system, such as IBM Corporation's 3995 Model 151 optical library with the capacity for as many as 144 double-sided optical disk cartridges and as many as four optical disk drives. Each DASD volume can be emulated with six optical disk surfaces (three disk cartridges); each disk surface emulates 5565 DASD tracks divided into 56 1024-byte sectors. Initializing each emulated track with the HA and RO fields requires about 42 minutes per surface or about 200 hours to initialize an entire 144-cartridge library--over eight days--during which the library is not available to the user. Using multiple drives in parallel can reduce the time but, even in a library having four optical drives, the initialization time is still about 50 hours.
Additionally, when data is to be read from a track on the disk, the entire 56-sector track is read into a track buffer, even if the data only occupies a single sector and the remaining sectors are unrecorded. Similarly, when data is written to a track, an image of the entire track is copied from the buffer, again even if the data only occupies a single sector and the remaining sectors are unrecorded. Consequently, data input/output efficiency suffers.